This invention relates to modulation monitoring and fault location of repeaters in submarine cable systems.
Submarine cable systems in which one cable is used for signal transmission in both the East to West and West to East directions along the cable, one signal being transmitted in a high frequency band and the other signal being transmitted in the low frequency band, require repeaters periodically spaced along the coaxial cable to reamplify the transmitted signals. Separate high band and low band amplifiers are employed in one type of repeater used in such coaxial cable systems. Since the repeaters and their associated amplifiers are inaccessible for testing once a submarine coaxial cable is laid, it is desirable to test repeater performance by applying test signals from the transmitting ends of the system. By examining the responses of the repeaters to the test signals, the performance of the individual repeaters can be determined. A well-known technique for measuring the performance of repeater amplifiers is to measure the magnitude of the modulation components generated when test signals at two distinct frequencies are applied to the inputs of repeater amplifiers. The repeater amplifiers intermodulate the applied test signals and generate modulation components which are indicative of the performance of the repeater amplifiers. Prior art repeater testing systems have required various component additions at each repeater to return a signal representative of the modulation performance of the repeaters to a transmitting end. This additional equipment has undesirably added to the cost and complexity of each repeater in the coaxial cable system.